1. Field of the Invention
The present invention relates to a band rejection filter capable of reducing harmonics and related satellite signal receiver, and more particularly, to a band rejection filter and related satellite signal receiver capable of avoiding magnetic saturation generating non-linear harmonic interference signal, to ensure transmission quality.
2. Description of the Prior Art
A purpose of a filter is to pass signals in a certain frequency band, and reduce signals outside of the frequency band. The filter can be divided into four categories: a low pass filter, a high pass filter, a band pass filter, and a band rejection filter according to functions of the filters. Because the band rejection filter can filter out signals in a certain frequency band, it is often utilized in a communication system, such as a satellite signal receiver, which transmits signals via a low frequency band or a high frequency band.
Please refer to FIG. 1, which is a schematic diagram of a satellite signal receiver 10. The satellite signal receiver 10 is preferably a superheterodyne receiver, which includes an antenna 100, a frequency down converter 102, and a set-top box 104. A satellite signal is received by the antenna 100, lowered to an intermediate-frequency (IF) signal by the frequency down converter 102, and then processed by the set-top box 104. A signal path between the frequency down converter 102 and the set-top box 104 is formed by a cable 106. In other words, the cable 106 transmits a direct-current (DC) power (with frequency 0 MHz) provided by the set-top box 104 to the frequency down converter 102, signals (with frequency 2.3 MHz) used by a two-way frequency shift keying communication between the frequency down converter 102 and the set-top box 104, and an IF signal (with frequency from 250 to 2150 MHz) outputted from the frequency down converter 102 to the set-top box 104. Generally, the cable 106 can transmit the DC power and related signals effectively, to ensure the frequency down converter 102 and the set-top box 104 to function normally. However, in some cases, for example, if a plurality of the set-top boxes 104 connects to each other through a home network, the cable 106 may affect the transmission quality of the home network. The main reason is a DC converter, a crystal oscillator, and a frequency shift keying (FSK) modulator, etc in the frequency down converter 102 may leak signals out during operations. When leaked signals fall to a frequency band (from 4 to 100 MHz) of the home network, the communication quality of the home network will be affected. In this situation, a band rejection filter is added between the frequency down converter 102 and the set-top box 104 (namely on the cable 106), and a rejection frequency band of the band rejection filter is at least 4 to 100 MHz, to filter out the leaked signals which may influence the home network.
In the prior art, there are many methods to realize the band rejection filter. The most common method is to combine a high pass filter and a low pass filter. Please refer to FIG. 2A, which is a schematic diagram of a band rejection filter 20 utilized in the cable 106 shown in FIG. 1, and FIG. 2B, which is a schematic diagram of a frequency response diagram of the band rejection filter 20. As shown in FIG. 2A, the band rejection filter 20 includes a high pass filter 200 and a low pass filter 202. In addition, as shown in FIG. 2B, a highest rejection frequency FH of the high pass filter 200 needs to be larger than 100 MHz and smaller than 250 MHz, to filter high frequency leaked signals out and ensure that IF signals outputted from the frequency down converter 102 can be transmitted to the set-top box 104. A lowest rejection frequency FL of the low pass filter 202 needs to be larger than 2.3 MHz and smaller than 4 MHz, to filter out low frequency leaked signals and ensure the DC power and the two-way FSK signals transmitted normally. The low pass filter 202 is usually composed by inductors and capacitors, and the inductors are usually coil magnetic-core inductors because of cost and size issue. However, since a power standard of a 2.3 MHz FSK signal is −4 dBm, a high power signal causes inside of the magnetic-core magnetic saturated and generating non-linear effect when passing a coil magnetic-core inductor. Thus, harmonics corresponding to 2.3 MHz, such as 4.6 MHz, 6.9 MHz, 9.2 MH, etc signals, will be generated, and affect communication quality of the home network if the harmonics fall in 4 to 100 MHz frequency band of the home network.